In recent years, governments have adopted emission standards for internal combustion engines which standards require lessening of harmful emissions from internal combustion engines. Initially, these standards were imposed upon gasoline fueled engines, and more recently standards are being adopted for diesel engines. Because of their durability and high power output, diesel engines are firmly established as the engine of choice in the trucking and off-the-road industries. There are over 150,000,000 diesel engines currently in operation worldwide, with over 10,000,000 new engines entering the market each year. In anticipation of these new standards which require reduced emissions, diesel engine manufacturers are developing and modifying the existing fuel management systems. With the next generation of engines utilizing radical changes in design not anticipated to be available until at least the year 2000, the only practical approach to meeting fuel emission standards without the sacrifice of fuel economy will have to be improvements in the existing designs of fuel management systems. At the present time, changes to the fuel management systems designed to meet the emissions requirements will add considerably to the cost of the engines or reduce fuel efficiency, or both. This is primarily because efforts to reduce emissions have been directed toward adding on to, or modifying, existing fuel system designs by incorporating higher precision pressure components and faster timing control devices. These efforts are directed specifically to control only one of the several factors of emissions--fuel volume efficiency.
However, when higher pressure is used to improve and control emissions, the precise pressure devices become very costly, and within some design configurations, become impractical and/or volume control efficiency is sacrificed. This is because known designs are co-dimensional in actual design concept and function. In other words, fuel pressure, timing and fuel volume are all interdependent and inter-related in the current designs. Moreover, there are practical limitations on engines with regard to pressure and timing when prior art fuel management systems are employed. Therefore, in part, attempts to modify and improve existing engine designs are restricted. This is because the over-all engine investment increases the need to improve the current fuel management systems.
There is therefore a need for technological improvements that will meet emission control standards, without change to existing basic engine designs, or without sacrifice of fuel volume efficiency.
All current designs of diesel engines utilize fuel-injection systems to meet the fuel metering requirements of the engine. These fuel injection systems include a variety of mechanical and electrical configurations designed to meter fuel to each cylinder in the most efficient manner. In most systems, mechanical components make up the greater cost, with electronic components being the least expensive to manufacture.
The mechanical components of a fuel management system are divided into two main groups, the low pressure components and the high pressure components.
The low pressure components consist generally of the fuel tank, fuel filter, fuel supply pump and/or lift pump. These low pressure components utilize standard precision parts, and in today's designs are generally acceptable from both a cost effectiveness and efficiency standpoint. At the current state of the art, even with modest increases in efficiency to the low pressure components, very little overall system efficiency is gained.
The components that make up the high pressure group include the injection pump, injector supply rail, overflow valves, high speed solenoid valves and injectors. These high pressure components have high precision and high reliability by design. Thus, these components represent a substantial part of the manufacturing cost of a fuel management system. Unlike the low pressure components, a small increase in efficiency in any one of the high pressure components can gain a modest increase in overall fuel management system efficiency.
Both the high pressure and low pressure components of the fuel system are integrated into the supply and return lines and are interfaced with the electrical monitoring devices through an electronic control module or "ECM". The function of the control module is to receive inputs from the engine and the operator and produce output commands to the controllable components of the fuel injection system.
The speed, reliability and cost of the primary electrical components far exceed the speed, .reliability and cost of the secondary electronic components or any part of the mechanical components of a fuel management system. In general, primary electrical components of a fuel injection system for current diesel engine designs, like the electronic control module, are adequate. However, the secondary electrical components, such as the electronically controlled solenoids within the high pressure group, are currently being upgraded in order to meet the co-dimensional demands. These electronically controlled solenoids are an integral part of the fuel injectors and the need for faster acting and lower power solenoids are necessary to improve performance and lower emissions.
Because of the co-dimensional dependencies of fuel pressure, volume and timing in current fuel management systems, most of the efforts to meet emission standards have been directed to both the mechanical and electrical components in the high pressure group. In recent years most of the improvements involved the co-dimensional relationship between timing and pressure, higher precision manufactured components, and faster acting solenoid valves, all of which resulted in small advancements in the fuel management systems for the diesel industry.
As an example of prior art improvements in fuel management systems, some such systems use a cam lobe, integral with the existing mechanical valve cam located on top of the engine. This cam lobe activates a plunger in the injector for producing the necessary pressure of injection. The injection pressure in such systems is controlled by the profile and the velocity of the cam lobe that drives a plunger downwardly through a cylinder inside the injector. The cam lobe forces the injector's plunger to travel the full injector stroke, thereby dispensing the full volume of the injector's cylinder during each injector cycle, with fuel constantly being internally bled off to the fuel tank through the solenoid valve. The precise timing of the injection is initiated by energizing the solenoid valve, thereby directing the fuel towards the injection nozzle instead of the tank, with fuel metering being produced by precisely timing the moment the solenoid is de-energized.
In these prior art systems, injection pressure is thus controlled by the leverage and speed of the cam profile at the time of injection and the internal size of the injector nozzle, while the volume of fuel injected is controlled by varying the "on-time" of the solenoid valve. As is true with other prior art systems, this system is co-dimensional, and thus the timing of the solenoid verses the cam profile and engine speed has to be precise to control the fuel metering.
Changing the timing of the injection also changes the point on the cam where injection occurs, and a given solenoid valve's "on-time" will result in a different amount of fuel delivered. This requires that different control values be programmed into the control module to compensate for timing, resulting in an "averaging" of the fuel metering. Moreover, when cam velocity changes, the rate of fuel forced through the injector nozzle changes, which also changes pressure in the nozzle, and thus injection pressure also becomes co-dimensional with timing and engine speed. Fuel volume efficiency is sacrificed due to injection variations caused by imperfect repeatability of the cam lobe, the control solenoids, and the control modules compensating for timing.
Other prior art systems use a high pressure common rail with a high pressure piston-rotary cam style pump that feeds high pressure fuel to the common fuel rail for storage prior to the injection event. Each fuel injector is connected to the common rail through a solenoid which, in conjunction with the control module, controls the injection timing and fuel volume.
Internally, the injector has a needle spool valve that is under high pressure on one end, and is activated when the opening of the solenoid valve creates an imbalance in pressure at the top of the needle, thus lifting the needle and allowing high pressure to flow through the nozzle chamber and into the engine.
The volume of fuel injected with this system depends on the stored high pressure and precise timing, and thus this system is co-dimensional. When higher pressure is required in order to reduce emissions, high pressure waves (Helmoltz resonance) occur throughout the high pressure components causing fuel metering problems, and precision fuel delivery is thus sacrificed in order to control emissions. Additional components and fuel metering "averaging" is added to the fuel management system to compensate for these high pressure waves. Moreover, high pressures and the high pressure waves subject the parts of the fuel system to design and durability problems.
During normal operation of a fuel injected engine, the same volume of fuel is not injected into all cylinders due to imbalances in the system and the co-dimensional dependencies that exist. This condition is evidenced at slow idle by a roughness in engine speed. At higher throttle settings this imbalance in delivery is evident as a loss of fuel efficiency. "Averaging" is the fuel system designers way of estimating composite fuel delivery to the entire engine. Injection timing and other parameters are therefore based on the average fuel delivered to each cylinder rather than the actual delivery rate on a cylinder to cylinder basis. Using this "averaging" principle, there is a known popular style of injection system which features a mechanical or electronic governor actuator pump that has an integral timing device dedicated to each of the pump's plungers which are mated to a specific injector. The pump generates the necessary pressure and distributes the fuel to the individual injectors, while a mechanical control collar or electric solenoid controls the quantity of the fuel that is injected. The metered fuel is "averaged" by means of the pump's governor mechanism, either mechanically or electrically. Fuel timing and injection pressure are controlled by the same piston's volume chamber, and prior to injection the rotation of the pump is matched with the rotation of the engine. Therefore, this system is also co-dimensional.
In all of these mechanical control systems of the prior art, many components must be manufactured to precise tolerances, while in the electrical/mechanical systems, fuel "averaging" is used to control actual metered fuel. Fuel volume efficiency is sacrificed due to timing variations of the electrically controlled governor.
Another prior art system utilizes a "medium" pressure fuel rail which pressure is then intensified in the injector. The solenoid for each injector is activated to enable medium pressure fuel to flow from the supply rail to the top of an intensifier piston in the injector. With an area difference between the top and the bottom of the intensified piston, pressure is increased and medium pressure fuel is intensified into high pressure fuel. This high pressure intensified fuel then flows through a check valve into a nozzle chamber and into the engine. Fuel metering is controlled by varying the "on time" of a solenoid that passes fuel into the top or medium pressure side of the intensifier piston. By using intensified injection, very few components are under high pressure. However, the system is, like all prior art systems, co-dimensional.
With all of the foregoing described prior art systems, the industry trend, in summary, is towards higher injection pressure of the fuel into the engine's piston chamber in order to meet future emissions standards. The problem with increasing the injection pressure, is that with a co-dimensional injection system, volume efficiency is sacrifice.
The primary emphasis of the industry is to improve, by redesign and increased cost, the components that depend directly on pressure and timing to control volume. Fuel "averaging" is the trend established by the industry to overcome most of the co-dimensional dependencies. This in itself is netting less then the desired fuel volume efficiency.
Moreover, additional fuel saving techniques like pilot injection, injection rate shaping, and inlet swirl become a secondary emphasis for fuel management systems.
There is therefore a need for a fuel management system that can utilize the best available components of current designs, while having the flexibility of adding fuel saving features that are currently known to improve emission standards without "averaging" and sacrificing fuel volume efficiency.